Two-layer external breast prosthesis with self-shaping feature and process for manufacture thereof

ABSTRACT

A two-layer external breast prosthesis comprising a first inner layer comprised of a self-shaping dispersion and a second outer layer comprised of an elastic material, wherein the second outer layer is relatively stiffer than the first inner layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of and claims priority toco-pending U.S. patent application Ser. No. 10/081,116, filed Feb. 21,2002, which application claims priority to U.S. Provisional PatentApplication Ser. No. 60/270,382, filed Feb. 21, 2001. The aforementionedapplication(s) are hereby incorporated by reference in their entiretyfor all purposes.

FIELD OF THE INVENTION

The invention relates generally to a breast prosthesis. Moreparticularly, the invention relates to a two-layer external breastprosthesis having a first outer layer filled with a relatively firmelastomeric material and an inner layer made of a flowable, self-shapingmaterial which is adapted to hold its shape against the chest wall of apatient.

BACKGROUND OF THE INVENTION

A variety of breast prostheses are known. These devices have beendeveloped so that women who have undergone a surgical procedure in whichthey have had one or possibly both breasts removed, for example amastectomy, may restore not only their cosmetic appearance aftersurgery, but their self-confidence as well. The use of breast prostheseshas thus minimized the stigma that may have otherwise been attached to awoman's appearance after breast removal surgery.

One example of a prosthetic breast is disclosed in German Patent No.DE-AS 26 05 148. This prosthesis has a shell shaped component enclosedor enveloped by plastic films, and is made of a two-component siliconerubber material. A cured two-component silicone rubber material is usedto form the shell of the prosthesis, which material corresponds in itsweight to the approximate weight of natural breast tissue. Thetwo-component silicone rubber material also simulates the actualconditions of a natural breast due to the soft, elastic responsivenessand mobility of the material. Additional breast prostheses of this type,which are intended for being placed in and supported by a brassiere, aredisclosed in U.S. Pat. Nos. 4,172,298 and 4,249,975, respectively.

In order to assure the most natural appearance of the breast prosthesiswhen inserted in the brassiere, a breast prostheses of the typedescribed above may be provided with a first body portion forming theconvex exterior side of the prosthesis, this portion being made of asoft set material. In addition, a second and flat shell shaped bodyportion forms the back or inner side of the prosthesis, which secondbody portion is made of a slightly harder set two-component siliconerubber material. The result of this design is that the elastically softmaterial of the first body portion, which due to its weight tends tosink slightly along the convex external side thereof, is supported bythe rear or flat shell body portion made of the stiffer material. Thedisadvantage of this type of prosthesis, however, is due to the factthat as the inner shell is fabricated of a relatively harder or stiffertwo-component silicone rubber material, the inner shell will press onthe sensitive scar tissue area(s) of the patient's external chest wall.Additionally, the inner shell also does not adjust, i.e., it does notconform, about the scar tissue area(s) in order to protect or cushionthose tissues.

The problem persists in that many of the known types of external breastprostheses have not been made adaptable to fit any irregularities thatmay be present in the chest wall of the mastectomy patient, particularlyas modern surgical techniques remove less tissue and leave moreirregular chest wall contours. Although matching a patient's chest wallcontours has been accomplished by the custom-molding of a prosthesis foreach patient, a need exists for a prosthesis that has desirableappearance and touch characteristics, which can adapt to the chest wallcontours of a wide variety of women, and can also be made available in acost-effective manner.

One attempt to satisfy this need is disclosed in U.S. Pat. No. 4,950,291to Mulligan, the provisions of which are incorporated herein by thisreference. The '291 patent discloses a 2-layer breast prosthesiscomprised of an outer layer or shell filled with a relatively firmelastomeric silicone gel, and an inner layer of an extremely soft andflowable cross-linked silicone gel. The softer, flowable gel willconform to the chest wall with a minimum of applied force, but does notpossess self-molding characteristics such that, for example, theflowable gel does not hold its shape when the prosthesis is removed.Also, due to the fact the inner gel readily flows, the gel can “pool” attimes in and along the bottom of the prosthesis due to the influence ofgravity, which in turn may lead to patient discomfort and generally doesnot offer any additional support to the fit and security of theprosthesis. Furthermore, the soft gel-like material disclosed in the'291 patent is of the same density as the outer gel, and can thereforeresult in a final product, which may be heavier than a comparable singlelayer prosthesis.

What is needed, but seemingly unavailable in the art, is an improvedbreast prosthesis that will have the desired outward cosmeticappearance, shape, and feel of a woman's breast, yet will be adaptableto the chest wall of the wearer by forming to the profile thereof. Whatis also needed is such an improved breast prosthesis which will belighter than the known prosthetic devices for greater patient comfortduring prolonged wear, and which may also be light enough that it willbe self-supporting on the chest wall of the wearer. Additionally, thereis a need for an improved two-layer breast prosthesis having arelatively firm outer shell component with a relatively soft andcompliant inner shell component that will not settle as the prosthesisis worn by a patient.

SUMMARY OF THE INVENTION

Among other aspects, this invention provides an improved and lightweightbreast prosthesis comprising two layers, each of which is a shell shapedcomponent. The breast prosthesis has an external component or bodyportion which mimics the shape of the breast with a hardness correlatingto the soft, elastic responsiveness of natural breast tissue, and has aninner component or body portion comprised of a flowable, self-shapingmaterial possessed with an ability to hold its shape.

In contrast to popular belief that the frontal area of the prosthesismust be supported by a rear shell-shaped component of a greaterhardness, the breast prosthesis of this invention fabricates this rearcomponent of a relatively softer material. The inner shell shapedcomponent thus is soft enough, due to its consistency, that it is ableto adjust to or fit about the scar tissue area(s) of the patient's chestwall without exerting any undue pressure thereon, and to maintain thatshape/conformation in the absence of internal shearing forces.

In a first aspect, the present invention provides a two-layer externalbreast prosthesis comprising a first inner layer comprised of aself-shaping dispersion, the dispersion having a specific gravity ofless than 1.0, a second outer layer comprised of an elastic material,wherein the second outer layer is stiffer than the first inner layer.

In a second aspect, the present invention provides a two-layer breastprosthesis comprising a first film envelope configured to define a firstinterior volume, and a second film envelope joined to the first filmenvelope along a common side edge to thereby define a second interiorvolume, wherein the second film envelope and the first film envelopeshare a common film wall. A first and a second sealed fill opening,respectively, extends from the common side edge of the first filmenvelope and the second film envelope to the respective first and secondinterior volumes. A first material positioned within the first filmenvelope forms an interior body portion of the prosthesis, the firstmaterial comprising a self-shaping material. A second material ispositioned within the second film envelope for forming an exterior bodyportion of the prosthesis. The second material comprises an elasticmaterial and/or curable elastic material precursor.

In still a third aspect, the present invention provides a process formanufacturing a two-layer breast prosthesis comprising the steps ofproviding a first film envelope configured to define a first interiorvolume, and a second film envelope joined to the first film envelopealong a common side edge to thereby define a second interior volume. Thesecond film envelope and the first film envelope share a commoninterstitial film wall, and both the first and second film envelopesfurther comprise a respective first and a second fill opening extendingfrom the common side edge of the respective first and second filmenvelopes to the respective first and second interior volumes. Themethod also comprises at least partially filling the second interiorvolume by passing an elastic material and/or curable elastic materialprecursor through the second fill opening; at least partially fillingthe first interior volume by passing a self-shaping dispersion throughthe first fill opening; and sealing the first and second fill openings.

Additional advantages and embodiments of the invention will be obviousfrom the description, or may be learned by practice of the invention.Further advantages of the invention will also be realized and attainedby means of the elements and combinations particularly pointed out inthe appended claims. Thus, it is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory of certain embodiments of the invention andare not restrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front or top plan view of a breast prosthesis embodying thepresent invention, illustrating the first body portion thereof.

FIG. 2 is a rear or bottom plan view, in partial perspective, of theprosthesis of FIG. 1, illustrating the second self-shaping body portionthereof.

FIG. 3 is a rear or bottom plan view, in partial perspective, of theprosthesis of FIG. 1, illustrating an impression made in and “held” bythe self-shaping body portion thereof.

FIG. 4 is a cross-sectional view of the prosthesis of FIG. 1,illustrating the first and second layers or body portions thereof.

DETAILED DESCRIPTION

The present invention may be understood more readily by reference to thefollowing detailed description and any examples provided herein. Theterminology used herein is used only for the purpose of describingparticular embodiments of the present invention and is not intended tobe limiting in any way. Furthermore, it must also be noted that, as usedin the specification and the appended claims, the singular form “a,”“an,” and “the” comprise plural referents unless the context clearlyindicates otherwise. For example, reference to a component in thesingular is intended to comprise a plurality of components.

Ranges may be expressed herein as from “about” or “approximately” oneparticular value and/or to “about” or “approximately” another particularvalue. When such a range is expressed, another embodiment comprises fromthe one particular value and/or to the other particular value.Similarly, when values are expressed as approximations, by use of theantecedent “about” or “approximately”, it will be understood that theparticular value forms another embodiment. Moreover, it should beunderstood that throughout this application, where publications arereferenced, the disclosures of these publications in their entiretiesare hereby incorporated by reference into this application in order tomore fully describe the state of the art to which this inventionpertains.

As indicated above, in a first embodiment, the present inventionprovides a two-layer external breast prosthesis comprising a first innerlayer comprised of a self-shaping dispersion having a specific gravityof less than 1.0, and a second outer layer comprised of an elasticmaterial, wherein the second outer layer is stiffer than the first innerlayer. In accordance with this and other embodiments disclosed herein,therefore, the self-shaping dispersion of the first inner layerpreferably comprises a dispersion of silicone oil and a plurality ofmicrospheres formulated to provide the desired self-molding or shapingcharacteristics. In one embodiment, the self-shaping dispersion has aspecific gravity of less than 1.0. In another embodiment, the dispersionpreferably has a reduced density of 0.4-0.5 (where water has a specificgravity of 1.0), compared to a silicone gel containing no microspheresand having a specific gravity of 1.0. Advantageously, the use of thisself-shaping dispersion is capable of providing the desired self-shapingcharacteristics while reducing the overall weight of the prosthesis.

In an alternate embodiment, it is contemplated that the self-shapingdispersion will be comprised of non-crosslinked material. As a result,the self-shaping material can advantageously be redistributed within thebreast prosthesis to accommodate special considerations on the chestwall, such as mounds or voids. Accordingly, the wearabilitycharacteristics of the breast prosthesis are substantially improved, anddespite the use of a softer, flowable, and formable material and designof the inner shell, the appearance of the breast prosthesis wheninserted in the brassiere does not suffer so that the prosthesis retainsits natural look.

To this end, in a preferred embodiment, the self-shaping material willcomprise a dispersion of microspheres suspended, or dispersed, within afluid media. It is anticipated that the fluid media will comprise asilicone oil having a viscosity in the range of from approximately 100CSt to approximately 5000 CSt, including such values as 200, 300, 400,500, 600, 700, 800, 900, 1000, 2000, 3000, and 4000. In one embodiment,the silicone oil preferably has a viscosity of approximately 500 CSt. Tominimize the separation of the dispersed components, i.e., themicrospheres, optional additives such as thixotropic agents andstabilizers may also be added to the fluid/dispersion mixture. The fluidmedia acts as a lubricant which allows the microspheres to shear andflow within the inner layer of the prosthesis when it is receivedagainst the patient's chest wall while being worn. After application, inthe absence of shearing forces, the weak mechanical interactions of themicrospheres create a weak structure which allows the material tomaintain its conformation, i.e., its shape. It should be understood thatsince the self-shaping dispersion is an uncured dispersion, it mayseparate over time, but can be regenerated by gently kneading thebackside of the inner layer of the prosthesis for a few seconds.

In accordance with this and other embodiments that will be describedbelow, it should also be understood that the plurality of microspherescan be comprised of a variety of polymeric substances known to one ofordinary skill in the art, including without limitation, such substancesas glass or acrylonitrile copolymers. Examples of available microspheresthat may be used in the self-shaping material include the Expancel091-DE-50; 091-DET-50; 091-DE-80; 091-DET-80 (Akzo-Nobel) and Scotchlitemicrospheres available from the 3M Company. In one embodiment, themicrospheres are preferably comprised of an acrylonitrile copolymer.

In yet another embodiment, it should also be understood that suitablemicrospheres according to the invention will preferably have an averageparticle size of between 40 and 125 microns, including such values as45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, and 120microns. To this end, it is understood that the plurality ofmicrospheres can be hollow and/or gas filled. Such gases include,without limitation, organic gases such as methane, ethane, propane,butane, isobutane and the like, as well as inert gases such as nitrogenand the like. As such, in an alternate embodiment of the presentinvention, at least a portion of the plurality of microspheres arepreferably filled with an isobutane gas.

In view of the foregoing and in accordance with the present invention, atypical formulation of the self-shaping material may thus be comprisedof a silicone oil (500 CSt) of 100 pbw, blended with hollow microspheresof 4.5-5.5 pbw. Due to variances in specific gravity and size gradientsbetween microsphere batches, one of ordinary skill in the art willappreciate that the actual blend ratio may vary. The self-shapingdispersion is formulated so that it stands on its own, with aconsistency like whipped cream or well beaten egg whites.

As previously indicated, in several alternate embodiments, theself-shaping dispersion may be formulated with the addition of one ormore optional rheological additives and/or thixotropes. The addition ofthe thixotrope increases the time required for the components within thedispersion to separate, which improves processing and therefore reducesthe need for the end user to frequently knead the product to re-dispersethe components. Non-limiting examples of suitable thixotropes caninclude Aerosil, available from Degussa; Cab-o-sil, available from theCabot Corporation; and Wacker HDK, available from Wacker Silicones.Therefore, a typical, non-limiting example in accordance with thisembodiment will comprise silicone oil (500 CSt) in the amount of 100pbw; suitable microspheres in the amount of 2-4 pbw; and Aerosil 200 inthe amount of 1-4 pbw (thixotrope).

The second outer layer of the prosthesis is comprised of a relativelystiff elastic material. To this end, the relatively stiffer outer layerof the prosthesis maintains the shape of the breast form and preventsoutermost films from wrinkling while at the same time remainingsufficiently soft to yield to the touch. In a preferred embodiment, theelastic material is comprised of a heat cured silicone gel. It is alsounderstood that in an alternate embodiment the outer layer silicone gelcan likewise comprise a material of reduced density, and hence a reducedweight through the addition of microspheres.

It will be appreciated that the preparation of the relatively stiffsilicone gel based elastic material forming the outer layer of theprosthesis is generally within the expertise of those skilled in theart. An appropriate silicone rubber is described in detail by Patzke andWohlfarth in their article entitled “Venetzungssysteme beimSiliconkautschuk,” published in the CHEMIKER-ZEITUNG 97^(th) year (1973)No. 4, pages 176-180. Furthermore, the relative stiffness of thesilicone gel material may be selected as desired to attain theadvantages described hereinabove. Preferably, the silicone rubbermaterial forming the outer layer will have a penetration measured in arange from 20.0 to 24.0 mm. Such values represent measurements with a“precision” penetrometer using a 15 gram cone having an aluminum tipwith a height of 0.6 inches, a base diameter of 0.33 inches, and aplastic cone body extending 1.13 inches from the tip base and having abase diameter of 2.56 inches.

In accordance with this and other embodiments of the present invention,the first inner layer and second outer layer of the two-layer breastprosthesis are each confined within a first film envelope and a secondfilm envelope, respectively. In a preferred embodiment, the first andthe second film envelopes share a common, interstitial film walltherebetween. Therefore, in one embodiment, the inner and outer layersof the two-layer prosthesis are separated by this interstitial filmwall. To this end, any suitable film composition known to one ofordinary skill in the art can be used with the present invention.However, in a preferred embodiment, the films used to form therespective film envelopes are polyurethane films.

As previously set forth above, in a second aspect, the present inventionfurther provides a two-layer breast prosthesis comprising a first filmenvelope configured to define a first interior volume, and a second filmenvelope joined to the first film envelope along a common side edge tothereby define a second interior volume, wherein the second filmenvelope and the first film envelope share a common film wall. A firstand a second sealed fill opening, respectively, which in each embodimentof the invention may comprise a bifurcated fill valve or opening havingseparate channels passing to each respective body portion of theprosthesis, each extend from the common side edge of the first filmenvelope and the second film envelope to the respective first and secondinterior volumes. A first material is placed within the first filmenvelope forming an interior body portion of the prosthesis, the firstmaterial comprising a self-shaping material. A second material is placedwithin the second film envelope to form an exterior body portion of theprosthesis, the second material comprising an elastic material and/orcurable elastic material precursor.

In a third aspect, the present invention provides a process formanufacturing a two-layer breast prosthesis comprising the steps ofproviding a first film envelope configured to define a first interiorvolume and a second film envelope joined to the first film envelopealong a common side edge to thereby define a second interior volume. Thesecond film envelope and the first film envelope share a commoninterstitial film wall, and the first and the second film envelopesfurther comprise a respective first and a second fill opening extendingfrom the common side edge of the respective first and second filmenvelopes to the respective first and second interior volumes. Theprocess further comprises at least partially filling the second interiorvolume by passing an elastic material and or curable elastic materialprecursor through the second fill opening, at least partially fillingthe first interior volume by passing a self-shaping dispersion throughthe first fill opening, and thereafter sealing the first and second fillopenings.

Referring now to the drawings, in which like reference charactersindicate like parts throughout the several views, a breast prosthesis 5is illustrated. The breast prosthesis is made of a first layer,envelope, chamber, or body portion 7 and a separate second layer,envelope, chamber, or body portion 8. The prosthesis can be manufacturedby providing a first plastic film 9 and a second plastic film 12, whichare used to form the film envelopes of the prosthesis. A third film 11,an interstitial film is sealably affixed by heat welding or any othermeans know to one of ordinary skill, between the exterior films 9, 12and along their respective and common side edges to provide a first anda second film envelope and leaving only a bifurcated fill opening 16 foreach respective envelope or chamber and extending from the common sideedge of the two film envelopes into the interior volume of the envelopesso fabricated, and which envelopes will become the two body portions ofthe prosthesis. In accordance with the embodiments described herein, thefilms 9, 11, and 12 are preferably polyurethane films.

The films are placed in a lower mold half (not illustrated) of one ofthe known types of molds having a surface design similar to the desiredshape of a breast. A two-component soft set silicone gel or othercurable elastic material precursor 13 is passed through one conduit ofthe bifurcated fill opening 16 in the welded films 9 and 11 to therebyat least partially fill the interior volume of the first film envelope,and to also form the convex breast-shaped exterior first body portion ofthe prosthesis. A self-shaping dispersion or material 15 is passedthrough a second conduit of the bifurcated fill opening 16 between thefilms 11 and 12 to thereby at least partially fill the interior volumeforming the second layer or body portion of the prosthesis as well asthe concave interior side thereof. The aforementioned conduits of thebifurcated fill opening 16, otherwise referred to as the first and thesecond fill openings in the film envelopes, are then sealed shut,simultaneously or individually, by heat welding or any other means knownto one of ordinary skill in the art. The mold is then closed about theformed prosthesis by an upper mold component (not illustrated) whoseshape corresponds with the rear or back side of the prosthesis, and heatis applied to the mold which causes the film to form to the desiredbreast form shape, and cures the silicone gel in the first or exteriorbody portion of the prosthesis.

Therefore, in one embodiment, the present invention thus provides abreast form having two chambers or shells 7,8 defined by the threepolyurethane films 9, 11, 12 joined together along and at their outerperipheries, as illustrated in FIG. 4. The self-shaping material 15within the second chamber 8 closest to the patient's chest wall iscomprised of a dispersion of a silicone fluid and expanded microspheresformulated to give the desired self-molding characteristics. Thisdispersion has a reduced density of 0.4-0.5 (where water has a specificgravity of 1.0), compared to the specific gravity of an unfilled (nomicrospheres added) silicone gel of 1.0. The use of thismaterial/dispersion therefore provides the desired self-shapingcharacteristics while reducing the overall weight of the prosthesis.

So formed, the self-shaping material 15 of the soft inner dispersion canflow into cavities of the patient's chest wall when being worn therebyin order to conform to the contours left in the chest wall aftersurgery. The “feel” of the breast form is realistic because the outerlayer can move to the touch by displacing some part of the soft innersilicone dispersion. The stiffness of the outer gel layer 13 can bevaried to the desired parameters by controlling the composition curableelastic material precursor, so that the thickness of the outer layer isdecreased as the stiffness thereof is increased. Therefore, thestiffness and thickness of the gel layer 13 can be adjusted to producethe desired “feel” of the prosthesis.

The self-shaping material 15 is preferably blended in a double planetarymixer, a ribbon mixer, or similar mixer types and under a vacuum toremove any dissolved air from the material. The pumping of the material15 into the second body portion, chamber or envelope 8 of the breastprosthesis should be carried out with a positive displacement pump in aclosed configuration that does not allow the material to be otherwiseaerated.

Although several embodiments of the invention have been disclosed in theforegoing specification, it is to be understood by those skilled in theart that many modifications and other embodiments of the invention willcome to mind to which the invention pertains, having the benefit of theteaching presented in the foregoing description and associated drawings.It is thus understood that the invention is not limited to the specificembodiments disclosed hereinabove, but rather is intended to cover suchalternatives, modifications, and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claims.To this end, one skilled in the art will appreciate that in practicingthe present invention, only reasonable and routine experimentation willbe required to optimize such variables and conditions as set forthherein.

1. A two-layer external breast prosthesis, comprising: a) a first innerlayer comprised of a self-shaping dispersion, said dispersion having aspecific gravity of less than 1.0; and b) a second outer layer comprisedof an elastic material, wherein said second outer layer is stiffer thansaid first inner layer.
 2. The prosthesis of claim 1, wherein theelastic material of said second outer layer comprises a silicon gel. 3.The prosthesis of claim 1, wherein the elastic material of said secondouter layer is heat cured.
 4. The prosthesis of claim 1, wherein theself-shaping dispersion of said first inner layer is uncured andcomprised of a silicone oil and a plurality of microspheres.
 5. Theprosthesis of claim 4, wherein the silicone oil has a viscosity ofapproximately 500 CSt.
 6. The prosthesis of claim 4, wherein theplurality of microspheres are comprised of an acrylonitrile copolymerand have an average particle size of from approximately 40 microns toapproximately 125 microns.
 7. The prosthesis of claim 1, furthercomprising a film wall separating said inner layer and said outer layer.8. The prosthesis of claim 1, wherein said inner layer is confinedwithin a first film envelope, and wherein said outer layer is confinedwithin a second film envelope.
 9. The prosthesis of claim 8, wherein thesaid first and second film envelopes share a common film walltherebetween.
 10. The prosthesis of claim 8, wherein said first and saidsecond film envelopes each comprise a polyurethane film.
 11. Theprosthesis of claim 1, wherein said inner layer is confined within afirst film envelope and is sufficiently soft to yield to the touch, andwherein said outer layer is sufficiently stiff to substantially preventwrinkling of a second film envelope confining said outer layer.
 12. Theprosthesis of claim 1, wherein said outer layer is sufficiently stiff togenerally retain the shape of a human breast.
 13. A two-layer breastprosthesis, comprising: a) a first film envelope configured to define afirst interior volume; b) a second film envelope joined to the firstfilm envelope along a common side edge to thereby define a secondinterior volume wherein said second film envelope and first filmenvelope share a common film wall; c) a first and a second sealed fillopening, respectively, extending from the common side edge of the firstfilm envelope and the second film envelope to the respective first andsecond interior volumes; d) a first material within said first filmenvelope forming an interior body portion of the prosthesis, said firstmaterial comprising a self-shaping material; and e) a second materialwithin said second film envelope forming an exterior body portion of theprosthesis, said second material comprising an elastic material.
 14. Theprosthesis of claim 13, wherein the self-shaping material of d) is aself-shaping dispersion comprised of a silicone oil and a plurality ofmicrospheres.
 15. The prosthesis of claim 13, wherein said self-shapingmaterial of d) is constructed and arranged to be yieldably shaped by apatient's chest wall.
 16. The prosthesis of claim 14, wherein therespective plurality of microspheres have an average particle size offrom 40 microns to 125 microns.
 17. The prosthesis of claim 14, whereinsaid microspheres are comprised of an acrylonitrile copolymer.
 18. Theprosthesis of claim 14, wherein said microspheres are filled withisobutane.
 19. The prosthesis of claim 13, wherein the first material ofd) has a specific gravity less than approximately 1.